This invention relates to digital telecommunications systems and in particular to an arrangement and method for transmitting asynchronous transfer mode (ATM) traffic.
A recent development in telecommunications technology has been the introduction of the asynchronous transfer mode (ATM) transmission technique. The asynchronous transfer mode (ATM) technology is a flexible form of transmission which allows various types of service traffic, e.g. voice, video or data, to be multiplexed together on to a common means of transmission, the traffic being carried in cells each having a header containing information that is employed for routing and for subsequent processing of the cell to recover the payload. The service traffic is adapted typically into 53 byte cells comprising 5 byte headers and 48 byte payloads such that the original traffic can be reconstituted at the far end of the ATM network. This form of adaptation is performed in the ATM adaptation layer (AAL). The technique allows large volumes of traffic to be handled reliably and efficiently.
A local area network (LAN) capable of supporting multimedia on demand applications over an ATM network is described by Ott et al. in NEC Research and Development, vol. 35 No. 4, Oct. 1, 1994, pages 36 to 373. An arrangement and method for communicating between devices coupled to an Ethernet network and an ATM network is described in specification No. U.S. Pat. No. 5,420,858.
A further recent development in ATM transmission has been the introduction of minicells for low bit rate users to reduce the cell assembly delays previously experienced by such users. Such minicells from a number of users can be multiplexed together and packed into a standard ATM cell for transmission over a common virtual channel. Typically, these low bit rate users are being provided with services, e.g. Internet or multimedia services from one or more service providers or Servcos. The volume of traffic of this nature is rapidly increasing as more and more customers are subscribing to the services.
A particular problem with minicell traffic is the quantity of overhead information that accompanies the traffic and which is used in the de-multiplexing and delineation process. With the rapidly increasing volume of this traffic there is an increasing risk of congestion. Further, this overhead information occupies bandwidth that might otherwise be allocated to revenue earning payload transmission.
It is an object of the invention to minimise or to overcome these disadvantage.
It is a further object of the invention to provide an improved method and apparatus for the delivery of services to user terminals.
According to one aspect of the invention there is provided a method of for transporting service (Servco) traffic between terminals in a network arrangement comprising a service access layer incorporating multimedia gateways for access by said terminals, an asynchronous network layer, a service layer incorporating adaptation layer switches each coupled to the network layer, and a transport layer providing a connection medium between said adaptation layer switches, the method including packetising the service traffic into minicells on ingress to the network layer, multiplexing the minicells to form the payload of ATM cells, transmitting the ATM cells in header-less form across the network layer, and de-multiplexing and recovering the minicells on egress from the network layer.
According to another aspect of the invention there is provided a network arrangement for transporting service (Servco) traffic between terminals coupled to the network, the arrangement comprising a service access layer incorporating multimedia gateways for access by said terminals, an asynchronous network layer, a service layer incorporating adaptation layer switches each coupled to the network layer, and a transport layer providing a connection medium between said adaptation layer switches, the arrangement being characterised by means for packetising the service traffic into minicells on ingress to the network layer and for multiplexing the minicells to form the payload of ATM cells, means for transmitting the ATM cells in header-less form across the network layer, and means for de-multiplexing and recovering the minicells on egress from the network layer.
The ATM stream takes the form of packets or mini-cells of variable but deterministic length, whose length is either explicitly coded within known positions in those packets, or may be implicitly derived from other known information in those packets, e.g. the circuit identifier for those packets. This implicit derivation may accommodate and depend on the state of the communications path or the state of the particular circuit within the communications path, which implies that the length may vary between an arbitrary multiplex of circuits of distinct users, or may also or separately vary per user. The length delineates the start of the subsequent packet of that communications stream. The packets contain in known fixed or derivable positions a linear systematic code employing redundancy to protect essential fields of those packets against error. The code also defines to a better than random probability the relative start boundary of same packet, which in conjunction with the length of the same packet can be used to correlate the linear systematic code word of all subsequent packets in the communications streams. This correlation may be assisted by the regular periodic insertion of a field whose contents form a linear systematic code word. This code word indicates the next or subsequent packet start boundaries to improve the performance of the delineation apparatus at the receiving equipment.
The ATM layer provides a cell of fixed size of 53 octets, the first five of which form a header that is constructed to be a linear systematic code word. The ATM layer may delineate these cell using methods described in for example in our United Kingdom specification No 2,232,040. Above the ATM layer is an adaptation layer designed to suit specific traffic types, of which one currently being proposed is ALL-CU or AAL-6. The essence of the AAL-6 proposals is to create a mini-cell, essentially a variable length packet with a fixed header at the start, which ensemble is appreciable shorter in length than the ATM cell to overcome the cell assembly delay which has been experienced in low-bit delay sensitive and real-time telephony applications.
The mini-cell is apportioned per user or multiplicity of users and carries its own circuit identifier in the header which is protected as a linear systematic code by parity check bits that may also form a class of cyclic code or equivalent code known to those skilled in the art. Many users"" mini-cells may be multiplexed into one ATM connection to overcome the cell assembly delay. Consequently, all these ATM cells carry the same header information. On a leased line application from a mobile base station transceiver to a mobile switching centre for example, only one ATM connection need be configured, in which case there is no replication of the same mini-cell circuit identifier being duplicated in a number of connections on the same communication channel and the ATM header becomes redundant.
Mini-cells must be separately delineated after the ATM cell stream has been delineated. A number of methods are possible that either allow the mini-cell header to be the sole means of delineation by virtue of the linear systematic code word of its header, or by using a pointer field at the start of each ATM cell payload that signifies the offset of the first mini-cell boundary in that ATM cell payload, or by ensuring that there is forced to be a mini-cell header at the start of every ATM cell payload, or by gathering together all the mini-cell headers to the end of the payload.
In the present invention, the ATM header is no longer required by virtue of its redundant contents and, because the mini-cell stream may be delineated independently of the ATM cell stream, this permits a greater bandwidth utilisation of the communications channel by allowing more mini-cells to be transmitted within a given bandwidth.